The solar-control stacks, also referred to as solar-protection stacks, to which the present invention relates, comprise functional layers that reflect infrared radiation, such as silver-based layers, combined with which are antireflection dielectric coatings which serve to reduce the light reflection and to control other properties of the stack such as the color, but which also serve as tie and protective coatings for the functional layers. The solar-control stacks commonly contain two functional layers surrounded by dielectric layers. More recently, stacks containing three functional layers, or even more than three functional layers, have been proposed in order to further improve the solar protection while retaining the highest possible light transmission. Each functional layer is spaced out by at least one dielectric coating so that each functional layer is surrounded by dielectric coatings. The various layers of the stack are, for example, deposited by sputtering under reduced pressure enhanced by a magnetic field, in a well-known device of magnetron type. The present invention is not however limited to this particular layer deposition process.
These solar-control stacks are used in the production of solar-protection glazings, in order to reduce the risk of excessive overheating, for example of an enclosed space having large glazed surfaces, due to sunshine and thus to reduce the air-conditioning load to be accorded in summer. The transparent substrate then often consists of a sheet of glass, but it may also, for example, be formed of a plastic film such as a PET (polyethylene terephthalate) film which is then enclosed between two sheets of glass by means of an adhesive polymer film such as a PVB (polyvinyl butyral) or EVA (ethylene/vinyl acetate) film in order to form a laminated glazing, or enclosed on the inside of a multiple glazing.
In this case, the glazing must let as little as possible total energetic solar radiation pass through, that is to say that it must have a relatively low solar factor (FS or g). It is however highly desirable for it to guarantee a certain level of light transmission (TL) so as to provide a sufficient level of illumination inside the building. These somewhat conflicting requirements express the wish to obtain a glazing having a high selectivity (S), defined by the ratio of the light transmission to the solar factor. These solar-control stacks also have a low emissivity which makes it possible to reduce the loss of heat by a long-wavelength infrared radiation. They thus improve the thermal insulation of large glazed surfaces and reduce energy losses and heating costs in a cold period.
The light transmission (TL) is the percentage of the incident light flux, of illuminant D65, transmitted by the glazing in the visible range. The solar factor (FS or g) is the percentage of incident energetic radiation which is, on the one hand, directly transmitted by the glazing and, on the other hand, absorbed by the latter then radiated in the direction opposite the source of energy with respect to the glazing.
These solar-protection glazings are in general assembled into multiple glazings such as double or triple glazings in which the sheet of glass bearing the stack is combined with one or more other glass sheets, which may or may not be provided with coating, the multilayer solar-control stack being in contact with the internal space between the glass sheets.
In certain cases, there is cause to carry out an operation for mechanically strengthening the glazing, such as a thermal toughening of the glass sheet or sheets, in order to improve the resistance to mechanical stresses. It is also optionally possible to be caused to give a more or less complex curvature to the glass sheets for particular applications, with the aid of a high-temperature bending operation. In the processes for manufacturing and forming the glazings, there are certain advantages in carrying out these heat treatment operations on the already coated substrate instead of coating an already treated substrate. These operations are carried out at a relatively high temperature, at which temperature the functional layer based on an infrared-reflecting material, for example based on silver, has a tendency to deteriorate and to lose its optical properties and its properties with respect to infrared radiation. These heat treatments especially consist in heating the glassy sheet at a temperature above 560° C. in air, for example between 560° C. and 700° C., and especially at around 640° C. to 670° C., for a duration of around 6, 8, 10, 12 or even 15 minutes depending on the type of treatment and the thickness of the sheet. In the case of a bending treatment, the glassy sheet may then be bent to the desired shape. The toughening treatment then consists in suddenly cooling the surface of the flat or curved glassy sheet with jets of air or coolant in order to obtain a mechanical strengthening of the sheet.
In the case where the coated glass sheet must undergo a heat treatment, it is therefore necessary to take very particular precautions in order to produce a stack structure which is capable of undergoing a toughening and/or bending heat treatment, sometimes referred to hereinbelow by the expression “toughenable”, without losing its optical and/or energy properties which give it its essential purpose. It is especially necessary to use dielectric materials, in order to form dielectric coatings, which withstand the high temperatures of the heat treatment without exhibiting a damaging structural modification. Examples of materials that are particularly suitable for this use are mixed zinc-tin oxide, and especially zinc stannate, silicon nitride and aluminum nitride. It is also necessary to make sure that the functional layers, for example based on silver, are not oxidized during treatment, for example by ensuring that there are, at the time of the treatment, sacrificial layers capable of oxidizing instead of the silver by capturing free oxygen.
It is also desirable for the glazings to meet certain esthetic criteria in terms of light reflection (RL), that is to say the percentage of the incident light flux—of the illuminant D65—reflected by the glazing in the visible range, and of color in reflection and in transmission. Market demand is for glazing with a light reflection that is moderate but not too low in order to avoid the “black hole” effect when looking at a facade under certain low light conditions. The combination of a high selectivity with a moderate light reflection sometimes leads to purple colors in reflection being obtained which are not very esthetic.
Solar-protection glazing is also used in the field of motor vehicle glazing, for example windshields but also other windows of the vehicle such as the side windows, rear windows or roof windows. In this field, the windows are often laminated, that is to say that the substrate bearing the stack is combined with another transparent substrate, that may or may not bear a stack, by means of an adhesive plastic film generally made of PVB, the solar-protection stack being positioned on the inside of the laminate in contact with the PVB. Vehicle windows must generally be curved in order to adapt to the shape of the vehicle. When the substrate is a sheet of glass, the bending operation is carried out at a high temperature and the substrate equipped with its stack is hence subjected to a heat treatment similar to the toughening treatment, with or without rapid cooling, described above with, in addition, a forming operation while the substrate is at high temperature.
In order to reduce the amount of heat which enters the premises or the vehicle through the glazing, the invisible infrared heat radiation is prevented from passing through the glazing by reflecting it. This is the role of the functional layers based on a material that reflects infrared radiation. It is an essential element in the solar-control stack. However, a large part of the heat radiation is also transmitted by visible radiation. In order to reduce the transmission of this part of the heat radiation and to go beyond the elimination of the input of energy by the infrared radiation, it is necessary to lower the level of light transmission.
Several solutions have been proposed to improve solar protection while retaining the maximum light transmission, but no solution provides a truly satisfactory glazing.
Patent application US 2009/0047466 A1 by German et al proposes a multiple glazing, one glass sheet of which bears a stack having three silver-based functional layers in which the first and last dielectric coatings comprise a dielectric absorbent material consisting of TiN or NbN. The intermediate dielectric coatings are transparent and do not contain any absorbent material. The color obtained in reflection on the glass side is not satisfactory because it is not neutral enough and tends toward purple which is a color that is not right from a commercial viewpoint. Furthermore, although the proprietor states that the color is relatively stable, it is observed in FIGS. 9 and 10 that the dispersion of points shows that the color varies quite a lot, both in reflection on the substrate side and on the stack side, or during a variation of the thicknesses of layers of the stack.
Patent application WO 2009/029466 A1 in the name of PPG Industries describes a laminated glazing for a motor vehicle in which a glass sheet bears a stack having three silver-based functional layers. The silver layers have a decreasing thickness from the glass sheet which bears them. This document describes a stack having a high light transmission which may be used to form a motor vehicle windshield. However, for low solar factors, for example of the order of 25%, the optical properties obtained do not meet the esthetic criteria that are desired from a commercial viewpoint, in particular the color in reflection is distinctly purple and unstable during a variation of the angle of observation. Moreover, the selectivity obtained is relatively low.
Patent application EP 645352 A1 filed by Saint-Gobain Vitrage describes solar-protection glazing, the stack of which comprises three layers of silver having an increasing thickness starting from the glass. The solar-protection double glazing comprising this stack has a solar factor of 30 or 34% according to examples 1 and 2 of the document. There is a commercial demand for obtaining lower solar factors, while retaining a maximum light transmission, in order to obtain a better solar protection. Furthermore, a high selectivity is only obtained at the expense of the stability of the color in reflection during industrial manufacture.